FT8
| FT8 | |
|---|---|
| Mode type | Digital weak-signal mode, 8-tone frequency shift keying |
| Developed by | Joe Taylor K1JT and Steve Franke K9AN |
| Released | 29 June 2017, in the WSJT-X software |
| Cycle | 15-second transmit and receive periods |
| Technical | |
| Signal bandwidth | About 50 Hz |
| Sensitivity | Decodes to −21 dB signal-to-noise in a 2.5 kHz bandwidth |
| Common frequencies | 7.074 MHz (40 m), 14.074 MHz (20 m), 21.074 MHz (15 m), 50.313 MHz (6 m) |
| Needs | A radio, a computer, a sound-card interface and accurate time |
FT8 is a digital mode that makes contacts with signals far too weak to hear. The radio is connected to a computer running the free WSJT-X program, which decodes every FT8 signal in the receiver's passband and shows them as lines of text. Contacts are short and almost entirely automatic: call signs, grid squares and signal reports are exchanged in fixed 15-second turns, and a complete contact takes about a minute. Since its release in 2017, FT8 has become the most popular mode on HF, with more contacts logged on it than on voice and CW combined.
Why it's so popular
- It works when nothing else does. FT8 decodes signals 21 dB below the noise floor, so a band that sounds dead on SSB is often full of FT8 activity. Hams with small antennas, low power or noisy city locations make worldwide contacts they could never make on voice.
- It's easy. Click a station in the list, and the software handles the exchange.
- It rewards awards chasers. Working all states, all continents or 100 countries is far quicker on FT8, and contacts can be confirmed electronically within hours.
The criticism is the other side of the same coin: there is no conversation. An FT8 contact exchanges call signs, four-character grid locators and signal reports, and nothing more. For many hams it is a tool for propagation checking and award chasing alongside other modes, not a replacement for them.
How it works
Each 15-second period, a transmitting station sends 12.6 seconds of audio tones, shifting among 8 tones 6.25 Hz apart, and the receiving software has 2.4 seconds to decode and queue a reply. Stations transmit in alternate periods, "even" or "odd". Messages are packed into 77 bits, which is why they are limited to call signs, grids and reports. Dozens of stations fit side by side in the 2.5 kHz audio passband of a single dial frequency, which is why everyone uses the same dial frequency on each band.
Because the timing is so tight, both computers must know the time to within a second or so. Internet time synchronization handles that.
Dial frequencies
FT8 activity gathers on one agreed frequency per band, the frequency shown on the radio's dial in upper sideband. The signals themselves sit within the 3 kHz audio passband above it, and the software finds them there. These are conventions that operators have settled on, not legal band edges; check the band plan for each band.
| Band | Dial frequency (USB) |
|---|---|
| 160 m | 1.840 MHz |
| 80 m | 3.573 MHz |
| 40 m | 7.074 MHz |
| 30 m | 10.136 MHz |
| 20 m | 14.074 MHz |
| 17 m | 18.100 MHz |
| 15 m | 21.074 MHz |
| 12 m | 24.915 MHz |
| 10 m | 28.074 MHz |
| 6 m | 50.313 MHz |
| 2 m | 144.174 MHz |
WSJT-X and similar programs come with these frequencies preset and can set the radio to them directly.
Getting on FT8
- Connect the radio's audio to the computer. Most modern transceivers do this over a single USB cable; older ones need a small sound-card interface.
- Install WSJT-X (or a derivative such as JTDX) and set the radio control, audio devices, your call sign and grid square.
- Set the radio to upper sideband on the FT8 dial frequency for the band, for example 14.074 MHz.
- Watch the waterfall and the decode window. Double-click a station calling CQ, and the software takes it from there.
Keep the power modest, typically 20 to 50 watts or less, and turn off the speech processor. Over-driving the audio produces a wide, distorted signal that interferes with the stations next to you and is the mark of a newcomer.
Relatives
FT8 is one of a family. FT4 is faster, with 7.5-second periods, for contests. JT65 and JT9 came earlier and are slower and more sensitive. WSPR is a beacon mode that reports where your signal is being heard, used to study propagation. MSK144 works meteor scatter on VHF, and Q65 is used for moonbounce.
See also
References
- FT8, Wikipedia.